In recent years, along with the progress in computer technology, there has been a rapid growth of techniques related to information storage apparatuses that are connected to a computer to perform the function of storing information. Among such information storage apparatuses, there is one that rotates a disk-shaped storage medium such as a magnetic disk and moves a head closely towards a surface of a storage medium, so as to record information on the storage medium and to reproduce information from the storage medium (hereafter the recording and reproducing operations will be collectively referred to as ‘access’). A hard disk drive (HDD) is a typical example of such an information storage apparatus, in addition to a well-known magneto-optical (MO) drive.
Higher recording density of storage media has been advanced in the field of information storage apparatus in recent years, and there is a strong demand for information storage apparatuses capable of accurately accessing such storage media with high recoding density. In the case of an information storage apparatus that accesses a storage medium by moving a head closely towards a surface of the storage medium, the access accuracy becomes higher as the distance between the head and the storage medium at the time of access become smaller. For this reason, in response to higher recording density of storage media, attempts have been actively made to reduce the distance between the head and the storage media at the time of access in the field of information storage apparatus.
The distance between a head and a storage medium at the time of access may differ among information storage apparatuses of even an identical model due to variations in the head and storage medium among the apparatuses. The variation in the distance is subtle, but such subtle variation may be a problem. Specifically, the subtle variation leads to differences in operational performance among the information storage apparatuses which have very short distances between their heads and storage media at the time of access and which are capable of accurate access. In order to solve such a problem, the following method has been conventionally employed for information storage apparatuses. Specifically, in each information storage apparatus, a heater which thermally expands the head is provided in the head. The heat amount generated by the heater is controlled so that the distance between the head and the storage medium is kept constant to be suitable for accurate access. In this method, before an information storage apparatus is made available for users, i.e., before shipment, association of the distances between the head and the storage medium, with the amounts of heat generated by the heater (hereafter, the heat amount will be referred also to as a ‘heater value’) is obtained. When the information storage apparatus is actually used by a user to access the storage medium, the heat amount by the heater is adjusted on the basis of the association so that the head is kept in a proper distance away from the storage medium. The association is determined by the following two steps. Firstly, the head is thermally expanded until the head comes in contact with (Touch Down) the storage medium. At this time, the distance between the head and the storage medium is considered to be zero. The heater value to accomplish this touch-down is determined as the TD heater value. Secondly, using the TD heater value as a reference, the association of the distances between the head and the storage medium, with the heater values is obtained for a range of the heater values not higher than the TD heater value. Various methods have conventionally been known as methods of determining the TD heater value (see, for example, Japanese Patent Application Publications No. 2006-190454, No. 2007-310978, No. 2008-47233, and No. 2008-112544). Some of those conventional practices of determining the TD heater value will be described below.
FIG. 1 is a flowchart illustrating an exemplary method of determining the TD heater value on the basis of changes in an automatic gain control (AGC) gain value.
The method of determining the TD heater values illustrated in FIG. 1 has the following processes. Firstly, an initial setting process is executed for the heater control to determine the TD heater value (step S101). Specifically, in this initial setting process, the heater value is set at the initial value zero. Besides, an increment, an maximum value and the like used when the heater value is gradually increased as will be described later are determined. After the initial setting process, a determination is made as to whether or not a current heater value is lower than the maximum heater value (step S102). The maximum heater value is sufficiently high at this time. Accordingly, it is determined that the current heater value is lower than the maximum heater value in this process (step S102: YES). Subsequently, the head reads data from predetermined data sectors of the storage medium (step S103). Reproduction signals that represent the read data are generated in this reading process. Many information storage apparatuses have a function to automatically amplify the signal level of the reproduction signal to a predetermined signal level, and a gain value obtained at this time is referred to as AGC gain value. Obtaining this AGC gain value is equivalent to obtaining a signal level of the original reproduction signal before amplified. The higher the signal level of the original reproduction signal preceding the amplification is, the smaller the AGC gain value becomes. This information storage apparatus that employs this method of determining the TD heater value generates the reproduction signal by reading the data at step S103, acquires the AGC gain value (step S104), and stores the AGC gain value thus obtained. After the storing of the AGC gain value, the heater value is increased by a predetermined increment (step S105). Next, a determination is made as to whether or not the AGC gain value stored at the previous step S104 reaches a saturation state (reaches a certain, constant value) or not (step S106). As will be described later, the AGC gain value is never saturated while the heater value is low. So, the first time the determination of the step S106 is made, ‘No’ is selected at the step S106. Subsequently, as long as the heater value is still lower than the maximum heater value (step S102: Yes), the processes from the step S102 to the step S106 are repeatedly executed while the heater value is gradually increased by the predetermined increment, so that the heater value becomes gradually larger.
In general, an increase in the heater value shortens the distance between the head and the storage medium, and results in higher reading accuracy so that a signal level of the reproduction signal obtained by the reading operation from the storage medium becomes higher. For this reason, the higher the heater value becomes, the smaller the AGC gain value for the reproduction signal becomes. Once the head has been brought into contact with the storage medium, a state may be seen that even a further increase in the heater value does not change the signal level of the reproduction signal any longer. In this state, the AGC gain value no longer changes (Saturation state of AGC gain value).
FIG. 2 illustrates the changes of the AGC gain value for the increase of the heater value in the method of determining the TD heater value of FIG. 1.
FIG. 2 illustrates the AGC gain values of the reproduction signals obtained as the heater value is gradually increased from zero by the above-mentioned predetermined increment. As FIG. 2 illustrates, the AGC gain value decreases in a substantially monotonic manner until right before 100 mW. Then right before 100 mW, the change in the AGC gain value becomes smaller than before, that is, the AGC gain value has reached to a saturation state.
In the method of determining the TD heater value of FIG. 1, whether or not the head contacts the storage medium is determined using the above-mentioned characteristic changes of the AGC gain value. Specifically, every time the heater value is incremented by the predetermined amount, a determination is made at step S106 as to whether or not the AGC gain value has reached to a saturation state. In this event, if the increase of the heater value by the predetermined increment causes only a smaller magnitude of change in the AGC gain value than a predetermined threshold, it is determined that the AGC gain value has reached to a saturation state (step S106 in FIG. 1: Yes). Then, the TD heater value is determined with the heater value for the previous AGC gain value marked just before the AGC gain value with the smaller magnitude of change than the predetermined threshold (step S107 in FIG. 1). FIG. 2 shows the three rightmost AGC gain values plotted near the heater value of 100 mW, in which a difference smaller than the predetermined threshold exists between the first rightmost AGC gain value for the heater value of 100 mW and the second rightmost AGC gain value that is adjacent to the first rightmost AGC gain value; and a magnitude of a difference larger than the predetermined threshold exists between the second rightmost AGC gain value and the third rightmost AGC gain value. Here, in the method of determining the TD heater value of FIG. 1, it is determined that, of these three AGC gain values, the first rightmost AGC gain value and the second rightmost AGC gain value are AGC gain values, which have reached to a saturation state, and the heater value of the third rightmost AGC gain value is determined as the TD heater value. In the above description, the heater value of the AGC gain value right before the AGC gain value reaches a saturation state (one right before) is determined as the TD heater value, however, the heater value of two right before the AGC gain value reaches the saturation state may be determined as the TD heater value. Which way of determination is adopted is set in advance in a stage of the initial setting process at step S101 in FIG. 1.
In the above-described example, the TD heater value is determined based on the AGC gain values for the reproduction signals obtained by reading information from predetermined data sectors, but, alternatively, the TD heater value may be determined based on the AGC gain values of the reproduction signals obtained by reading information from predetermined servo sectors. Also in this case, the TD heater value is determined through following series of processes similar to those in FIG. 1.
Next, a method of determining the TD heater value different from the method of determining the TD heater value of FIG. 1 will be obtained.
FIG. 3 is a flowchart illustrating an exemplary method of determining the TD heater value through a change in the Viterbi trellis margin (VTM) value.
In the method of determining the TD heater values illustrated in FIG. 3, similar to the method of determining the TD heater values illustrated in FIG. 1, firstly, an initial setting process is executed for the heater control related to determining the TD heater value (step S201), the heater value is set at the initial value, zero. Besides, the increment by which the heater value is gradually increased little by little and the maximum value that the heater value is increased up to or the like are determined in this initial setting process. After the initial setting is performed, it is determined as to whether or not the current heater value is lower than the maximum heater value (step S202). The maximum heater value is sufficiently high, and at this stage, it is determined that the current heater value is lower than the maximum heater value (step S202: YES). Subsequently, the head reads data from a predetermined data sector of the storage medium (step S203). A reproduction signal representing data are for a reading target generated by this reading process.
The information storage apparatus that employs the method of determining the TD heater value illustrated in FIG. 3 has a function to analyze part of the reproduction signals to obtain, on the basis of the Viterbi algorithm, a maximum-likelihood demodulated value of the part of reproduction signals. In the information storage apparatus that employs the method of determining the TD heater value, uses this function to compute the VTM values, which are values representing the degrees of offset between the maximum-likelihood demodulated values and their actual demodulated values, and the VTM values thus obtained are stored (step S204). The more the reading errors take place when the data is read from the data sector, the larger the VTM values become. If the reading errors take place too frequently, the VTM values diverge.
After the VTM values is stored, the heater value is incremented by a predetermined increment (step S205). Then, it is determined as to whether or not the VTM values thus stored at the previous step S204 are diverging (step S206). As will be described in detail later, the VTM values are never diverging in a state where the heater value is still low. So, the first time the determination of the step S206 is made, ‘No’ is selected at the step S206. Subsequently, as long as the heater value is lower than the maximum heater value (step S202: Yes), the processes from the step S202 to the step S206 explained above are repeatedly executed while the heater value is increased by the predetermined increment, so that the heater value becomes gradually larger.
In general, as the heater value is increased so that the distance between the head and the storage medium is shortened, reading accuracy increases and less reading errors take place. However, when the head has been brought into contact with the storage medium, the head collides, irregularly and repeatedly, the unevenness formed on the surface of the storage medium, so that the occurrence of reading errors becomes more frequently and the VTM values increase rapidly (the VTM value divergence).
FIG. 4 illustrates the changes in the VTM value along with the increase in the heater value in the method of determining the TD heater value of FIG. 3.
FIG. 4 illustrates the VTM values of the reproduction signals obtained as the heater value is gradually increased from zero by the above-mentioned predetermined increment. As FIG. 4 illustrates, the VTM value decreases at a moderate pace until the heater value reaches a value that is slightly smaller than 100 mW. When the heater value nearly reaches approximately 100 mW, the VTM value increases rapidly, which depicts that the VTM value has reached to a divergence state.
In the method of determining the TD heater value of FIG. 3, whether or not the head contacts the storage medium is determined every time the heater value is increased by the predetermined amount using such characteristic in the step 206. Specifically, every time the heater value is incremented by the predetermined amount, a determination is made at step S206 as to whether or not the VTM value reaches to the divergence state. In this event, if a magnitude of change in the VTM value when the heater value is increased by the predetermined increment becomes greater than a predetermined threshold, it is determined that the VTM value reaches the diverging state (step S206 in FIG. 3: Yes). Then, a heater value just before VTM value increases rapidly is determined as the TD heater value (step S207 in FIG. 3). In FIG. 4, a magnitude of change between the rightmost VTM value when the heater value is 100 mW and the second VTM value from the right adjacent to the VTM value for the 100 mW heater value is greater than the predetermined threshold, and, it is determined that the second VTM value from the right is determined as the TD heater value. In the description above, the heater value just before the VTM value increases rapidly is determined as the TD heater value. However, a heater value for the second last VTM value preceding the first diverging AGC gain value may be determined as the TD heater value. Which method of determination is adopted is set in advance in the stage of the initial setting process at step S201 in FIG. 3.
As described above, collide withherto, various conventional methods of determining the TD heater value have already been known, and conventionally, the detection of the contact of the head with the storage medium and the determination of the TD heater value are performed based on only one of those already known determination methods. Note that, in general, according to environmental factors such as the shape of the surface of the storage medium, the temperature, and the humidity frequently bring contacting of the head and the storage medium in different manners. Accordingly, there are quite a few cases where the adopted one of the methods of determining the TD heater value may not clearly distinguish a contact state and a non-contact state. For example, there are cases where, even when the head contacts the storage medium, the head keeps a low friction state with the storage medium and is slowly pressed onto the storage medium along as in the heater value is increased. Such cases are known as those of soft landing. In the soft-landing case, even when the head is actually in contact with the storage medium, the increasing of the heater value by the predetermined increment frequently cause the VTM value to have only a moderate amount of change. So, according to the method of determining the TD heater value of FIG. 3, the determined TD heater value is sometimes a little larger than the actual TD heater value, or, in some cases, it is erroneously determined that the head is not in contact with the storage medium.
If the TD heater value is inaccurately determined as in the above-described cases, the association of the heater value with the distance between the head and the storage medium is also inaccurate. In some cases, such inaccurate association brings the head into frequent contacts with the storage medium at the time of access, and such frequent contacts may even physically damage the storage medium. For this reason, it is desirable that the determination of the TD heater value be so accurate that the contact between the head and the storage medium is correctly determined irrespective of the manner of the contact between the head and the storage medium.
In view of the above-described circumstances, the control device, the control method, and the information storage apparatus of this disclosure aims to identify the TD heater value accurately.